This application proposes a continuation of a program of research investigating sensory and perceptual processes in spoken communication. The research aims to develop an empirical description and a principled account of the perceptual organization of speech. This perceptual function establishes the coherence of the speech signal despite its acoustic diversity and complexity. By resolving auditory activity into vocal sources, this fundamental function determines the sensory constituents in the perceptual analysis of the linguistic properties of speech. Four projects are proposed for the coming five years. The new research will develop original measures of perception and refine the empirical description of the perceptual organization of speech. In one, the experiments will estimate the temporal grain of auditory sensory integration in speech. In a second, the stability and effectiveness of modulation sensitivity will be estimated, in the auditory modality and in audiovisual speech perception. In a third, the enhancement of speech perception brought about by familiarity with a specific individual talker will be probed. In a fourth, an explanation will be sought for the causes of the uncertain effects of natural quality on speech perception. Overall, the empirical projects are a necessary prelude to formal and neurobiological characterization of the cognitive resources that insure the perceptual stability of spoken communication in natural environments, when the source of speech is visible or not, when the talker is familiar or not, and when the quality of the sensory samples of speech is natural or not. In emphasizing the perceptual utility of coarse-grain spectrotemporal properties, these studies can begin to explain the robustness of speech perception when auditory qualities are altered or diminished by disease, and offers an opportunity to identify therapeutic accommodations for failure or loss of perceptual resources.